Kinetics of nitrous oxide (N₂O) formation and reduction by Paracoccus pantotrophus

Nitrous oxide (N₂O) is a powerful greenhouse gas emitted from wastewater treatment, as well as natural systems, as a result of biological nitrification and denitrification. While denitrifying bacteria can be a significant source of N₂O, they can also reduce N₂O to N₂. More information on the kinetics of N₂O formation and reduction by denitrifying bacteria is needed to predict and quantify their impact on N₂O emissions. In this study, kinetic parameters were determined for Paracoccus pantotrophus, a common denitrifying bacterium. Parameters included the maximum specific reduction rates, [Formula: see text], growth rates, [Formula: see text], and yields, Y, for reduction of NO₃^- (nitrate) to nitrite (NO₂^-), NO₂^- to N₂O, and N₂O to N₂, with acetate as the electron donor. The [Formula: see text] values were 2.9 gN gCOD^-1 d^-1 for NO₃^- to NO₂^-, 1.4 gN gCOD^-1 d^-1 for NO₂^- to N₂O, and 5.3 gN gCOD^-1 d^-1 for N₂O to N₂. The [Formula: see text] values were 2.7, 0.93, and 1.5 d^-1, respectively. When N₂O and NO₃^- were added concurrently, the apparent (extant) kinetics, [Formula: see text], assuming reduction to N₂, were 6.3 gCOD gCOD^-1 d^-1, compared to 5.4 gCOD gCOD^-1 d^-1 for NO₃^- as the sole added acceptor. The [Formula: see text] was 1.6 d^-1, compared to 2.5 d^-1 for NO₃^- alone. These results suggest that NO₃^- and N₂O were reduced concurrently. Based on this research, denitrifying bacteria like P. pantotrophus may serve as a significant sink for N₂O. With careful design and operation, treatment plants can use denitrifying bacteria to minimize N₂O emissions.